Universal Mobile Telecommunications System (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in Wideband Code Division Multiple Access (WCDMA) based on European systems, Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS). The Long Term Evolution (LTE) of UMTS is under development by the 3rd Generation Partnership Project (3GPP) which standardized UMTS. There are many technical specifications hosted at the 3GPP website relating to Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN), e.g., 3GPP TS 36.300. The objective of the LTE work is to develop a framework for the evolution of the 3GPP radio-access technology towards a high-data-rate, low-latency and packet-optimized radio-access technology. In particular, LTE aims to support services provided from the packet switched (PS)-domain. A key goal of the 3GPP LTE technology is to enable high-speed packet communications at or above about 100 Mbps.
FIG. 1 illustrates an example of an LTE type mobile communications system 10. An E-UTRAN 12 includes E-UTRAN NodeB (eNodeBs or eNBs) 18 that provide E-UTRA user plane and control plane protocol terminations towards the user equipment (UE) 20 over a radio interface. Although an eNB is a logical node, often but not necessarily implemented by a physical base station, the term base station is used here to generally cover both logical and physical nodes. A UE is sometimes referred to as a mobile radio terminal and in an idle state monitors system information broadcast by eNBs within range to inform itself about “candidate” base stations in the service area. When a UE needs access to services from a radio access network, it sends a request over a random access channel (RACH) to a suitable eNB, typically an eNB with the most favorable radio conditions. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of the S1 interface to an Evolved Packet Core (EPC) 14 which includes a Mobility Management Entity (MME) by an S1-MME and to a System Architecture Evolution (SAE) Gateway by an S1-U. The MME/SAE Gateway are referenced as a single node 22 in this example. The S1 interface supports a many-to-many relation between MMEs/SAE Gateways and eNBs. The E-UTRAN 12 and EPC 14 together form a Public Land Mobile Network (PLMN). The MMEs/SAE Gateways 22 are connected to directly or indirectly to the Internet 16 and to other networks.
In order to enable operation in different spectrum allocations, for example to have a smooth migration from existing cellular systems to the new high capacity high data rate system in existing radio spectrum, operation in a flexible bandwidth is necessary, e.g., bandwidths ranging from 1.25 MHz to 20 MHz for downlink transmissions from network to UE. Both high rate data services low rate services like voice must be supported, and because 3G LTE is designed for TCP/IP, VoIP will likely be the service carrying speech.
LTE uplink transmission is based on so-called Discrete Fourier Transform Spread-OFDM (DFTS-OFDM) transmission, a low-peak to average power ratio (PAPR), single-carrier (SC) transmission scheme that allows for flexible bandwidth assignment and orthogonal multiple access not only in the time domain but also in the frequency domain. Thus, the LTE uplink transmission scheme is also often referred to as Single-Carrier FDMA (SC-FDMA).
The LTE uplink transport-channel processing is outlined in FIG. 2. A transport block of dynamic size is delivered from the media access control (MAC) layer. A cyclic redundancy code (CRC) to be used for error detection at the base station receiver is calculated for the block and appended thereto. Uplink channel coding is then performed by a channel encoder which may use any suitable coding technique. In LTE, the code may be a turbo code that includes a Quadratic Permutation Polynomial (QPP)-based internal interleaver for performing block interleaving as part of the turbocoder. The LTE uplink hybrid-Automatic Repeat Request (ARQ) extracts, from the block of coded bits delivered by the channel coder, the exact set of bits to be transmitted at each transmission/retransmission instant. A scrambler scrambles the coded bits on the LTE uplink (e.g., bit-level scrambling) in order to randomize the interference and thus ensure that the processing gain provided by the channel code can be fully utilized.
To achieve this randomization of the interference, the uplink scrambling is mobile terminal-specific, i.e., different mobile terminals (UEs) use different scrambling sequences. Terminal-specific scrambling also provides the scheduler the freedom to schedule multiple users on the same time-frequency resource and rely on base station receiver processing to separate the transmissions from the multiple users. Terminal-specific scrambling randomizes the interference from other mobile terminals in the same cell that happen to be scheduled on the same resource and improves the performance.
After scrambling, the data is modulated to transform a block of coded/scrambled bits into a block of complex modulation symbols. The set of modulation schemes supported for the LTE uplink example include QPSK, 16QAM, and 64QAM, corresponding to two, four, and six bits per modulation symbol, respectively. The block of modulation symbols is then applied to a DFTS-OFDM modulator, which also maps the signal to an assigned radio resource, e.g., a frequency sub-band.
Together with the modulated data symbols, the signal mapped to the assigned frequency band also contains demodulation reference signals. Reference signals known in advance by both the mobile terminal (UE) and the base station (eNodeB) and are used by the receiver for channel estimation and demodulation of the data symbols. Different reference signals can be assigned to a user terminal for similar reasons terminal-specific scrambling codes may be used, i.e., to intelligently schedule multiple users on the same time-frequency resource and thereby realize so-called multi-user MIMO. In case of multi-user MIMO, it is up to the eNodeB processing to separate the signals transmitted from the two (or more) UEs simultaneously scheduled on the same frequency resource in the same cell. Terminals simultaneously scheduled on the same frequency resource are typically assigned different (e.g., orthogonal) reference signal sequences in order for the eNodeB to estimate the radio channels to each of those UEs.
A basic requirement for any cellular or other radio communications system is providing a user terminal the capability to request a connection setup. This capability is commonly known as random access and serves two main purposes in LTE, namely, establishment of uplink synchronization with the base station timing and establishment of a unique user terminal identity, e.g., a cell radio network temporary identifier (C-RNTI) in LTE, known to both the network and the user terminal that is used in communications to distinguish the user's communication from other communications.
But during the (initial) random access procedure, uplink transmissions from the user terminal cannot employ terminal-specific scrambling sequences or reference numbers to randomize interference because the initial random access request message from the user terminal has just started communicating with the network and neither a terminal-specific scrambling code nor a terminal-specific reference number has net-been assigned to that user terminal. What is needed is a mechanism that permits random access messages sent over a shared uplink channel to be scrambled until a terminal-specific scrambling code can be assigned to the user terminal. One reason to scramble random access messages is to randomize inter-cell interference, which is also the case for scrambling during “normal” uplink data transmission. Scrambling can also be used to suppress intra-cell interference in case of multiple UEs being scheduled on the same time-frequency resource. Similarly, it would also be desirable for user terminals to transmit known reference signals during random access to allow the base station receiver to estimate the uplink channel. Reference signals need to be included in the random access messages as well as in “normal” uplink data transmissions to enable channel estimation at the eNodeB and corresponding coherent demodulation.